Sealed lead-acid cells and batteries, for many application, have significant advantages in comparison to the use of conventional, flooded lead-acid cells and batteries. Such cells and batteries, sometimes termed "VRLA" cells and batteries (i.e., valve-regulated, lead-acid), utilize bunsen valves and the like to maintain the desired internal pressure for an efficient oxygen recombination cycle.
Some applications where such sealed cells and batteries are used are termed stationary battery applications. In such applications, such cells and batteries are maintained at a full state-of-charge and in a ready-to-use condition, typically by floating at a constant preset voltage. Stationary cells and battery applications include use for stand-by or operational power, including telecommunications, utilities, emergency lighting for commercial buildings, stand-by power for cable television systems, and uninterruptible power supplies.
Such uninterruptible power supplies concern, for example, systems which back-up computers and communication networks. Having a reliable uninterruptible power source accommodates the orderly shutdown of computers when there is a sudden interruption in the primary power source, typically during an electrical power outage. Such an uninterruptible power source will also accommodate short, or intermittent, losses in power. In the event of a power interruption, the uninterruptible power source is subject to a rapid, and sometimes deep, discharge.
Another potential application for sealed lead-acid cells and batteries is a variety of motive power applications in which an array of cells or batteries provides the motive power for vehicles ranging from Class 1 to Class 3 trucks, various automated guided vehicles, mining vehicles and also railroad locomotives. The performance requirements for motive-powered vehicles are quite different from the performance requirements for stationary battery power sources. In stationary applications, the depth of discharge in service is relatively shallow, and the number of discharges is small, as most batteries are in float service. In direct contrast, motive power applications require relatively deep depths of discharge to be achieved on a continuous cycling basis over a period of time. Indeed, a common requirement for Class 1-3 trucks is that, in an 8-hour shift, the cell or battery assembly must be capable of delivering an 80% depth of discharge and that performance is required for about 300 cycles per year with a useful service life under those conditions of 4 or 5 years.
A common problem encountered by such VRLA cells and batteries is the integrity of the terminal seal over time as a result of grid growth which occurs in service. As has been especially common at the positive terminal, grid growth occurs as a battery grid corrodes over time, hence causing the battery terminal to move outwardly relative to the battery cover. This relative rise causes stress on both the required terminal-cover seal, as well as the requisite container-cover seal. As the container and cover are commonly made of plastic, each was known to fail in various ways, such as by fracturing, cracking at welds, and so forth. Failures such as these have resulted in the leakage of electrolyte from the cells and batteries and has also resulted in failure to operate properly. This well-known problem is particularly acute in sealed lead-acid cells and batteries because relatively high internal temperatures in service can be reached. Such relatively high temperatures lead to an increase in the rate of grid corrosion which can result in significant grid growth.
This problem is not only well-known, but has also commanded substantial attention. A wide variety of attempted solutions have been proposed.
U.K. Patent 2026761A thus describes an assembly for securing a terminal post in a cover of an accumulator. The terminal post has a shank with circumferential ribs onto which a collar member is molded. The member has an upwardly-extending, annular neck by which the member is welded to a corresponding neck of the cover. The welded neck of the cell cover is joined to the cover by a flexible annular part made of a thin thermoplastic material in corrugated form.
Another attempt to solve the problems caused by terminal expansion is illustrated in U.S. Pat. No. 4,445,356 to Barrette, Jr. The '356 patent depicts a battery having a plastic bellows integrally molded or heat or sonically sealed to a battery cover at one end and either molded, heat or sonically sealed to a lead terminal extension or a plastic terminal sleeve at another end, wherein the plastic bellows is raised above the level of the battery cover and the terminal post passes through the plastic bellows.
Still another attempt to solve the terminal expansion problem is shown by U.S. Pat. No. 4,467,021 to Stocchiero. This patent utilizes a flexible, concave battery lid for distributing stress due to elongation of terminal poles, and the poles are shown welded to a bushing. As noted in his later patent, U.S. Pat. No. 4,898,795, Stocchiero indicates that the lid in his '021 configuration presents limited elasticity where its width is considerably reduced in relation to its length.
Yet another attempt to solve the problems caused by terminal expansion is disclosed in the aforementioned '795 patent. The '795 patent depicts an elastic diaphragm co-axially aligned with an accumulator pole below the surface of the lid. The elastic diaphragm is shown as having either a corrugated, bellows-shaped tube or toroidal shape. In one embodiment, a threaded bushing receives a threaded pole. In this same embodiment, the diaphragm is integrally attached to the lid at one edge, and as the bushing is tightened around the pole, the other edge of the diaphragm is locked between the threaded bushing and an O-ring, which O-ring is positioned between the threaded bushing and the pole. As opposed to the proposed solutions previously described wherein diaphragms were designed to operate by flexible expansion, the diaphragm in the '795 patent is intended to operate by collapsing.
Despite all of the prior efforts in this area, there still exists the need for a cover-seal configuration which can adequately accommodate the inevitable grid growth in use, but which also can be readily manufactured. A satisfactory configuration thus must be capable of being efficiently made while achieving the reliable and requisite seals.
Accordingly, it is a primary object of the present invention to provide sealed lead-acid cells and batteries whose integrity is not compromised in service as the inevitable grid growth takes place.
Another object of this invention is to provide a flexible, yet durable, cell or battery cover which is responsive to terminal post expansion so that, as grid growth occurs, the cover responsively expands, maintaining its integrity, and that of the terminal post-cover seal.
A still further object of the present invention is to provide a method of manufacturing a cell or battery cover which is both efficient and also results in a reliable configuration.
Other objects and advantages of the present invention will be apparent as the following description proceeds, taken with the accompanying drawings.